Nonaqueous electrolyte solution secondary batteries, especially lithium ion secondary batteries, each of which has a high energy density, have been widely used as batteries for use in, for example, a personal computer, a mobile phone, and a portable information terminal.
A nonaqueous electrolyte solution secondary battery, typified by a lithium ion secondary battery has a high energy density. Thus, in a case where an internal short circuit occurs due to, for example, a breakage in the battery or penetration of a foreign matter, a large electric current flows and the battery generates a high heat. This requires a nonaqueous electrolyte solution secondary battery to have a function of preventing generation of heat having a level equal to or higher than a certain level. Known examples of a nonaqueous electrolyte solution secondary battery having such a function include a battery including a separator having a shutdown function. The shutdown function is a function of causing a separator to block passage of ions between a cathode and an anode during abnormal heat generation. This function makes it possible to prevent further heat generation.
Proposed examples of such a separator having the shutdown function include a porous film including a shutdown layer containing polyolefin as a main component; a porous film including a shutdown layer containing polyolefin as a main component and a heat-resistant layer containing a heat-resistant resin (Patent Literature 1); a porous film including a shutdown layer containing polyolefin as a main component and an adhesive layer containing an adhesive resin (Patent Literature 2); and the like.
Since a porous film is porous and has a large surface area in which water adheres to the porous film, the porous film easily contains moisture. It is known that a separator which contains much moisture causes dissolution of the moisture into an electrolyte solution and promotes generation of, for example, hydrogen fluoride and decomposition of the electrolyte solution. Thus, an amount of moisture contained in a separator is preferably maintained at a level as low as possible.
It is known that moisture contained in a porous film is removed by, for example, vacuum drying or heat drying.